1. Field of the Invention
The present invention relates to a semiconductor light detector utilizing an avalanche effect and, more particularly, to a semiconductor light detector having an improved guard ring structure and a method of manufacturing the same.
2. Description of the Related Art
In recent years, an avalanche photo diode (to hereinafter be referred to as an APD) has been used as a high-sensitivity light detector in such fields as optical fiber communication systems. An APD is a device utilizing avalanche-carrier multiplication of a semiconductor crystal, and has an avalanche region having a high electric field inside the device. For this reason, a guard ring structure is important to prevent local breakdown caused by electric field concentration in the avalanche region; in particular, in the peripheral portion of the avalanche region.
An optical communication system generally uses an APD which includes a light detection layer (light absorption layer) consisting of GaInAs (having a composition lattice-matched with InP) having high sensitivity in a low-loss range (wavelength; 1.3 to 1.5 .mu.m) of a quartz optical fiber, carriers generated in the light receiving layer being subjected to avalanche multiplication by InP.
Since a high electric field region necessary for avalanche multiplication is formed at a front portion of a p-n junction having a sharp concentration gradient (to hereinafter be referred to as a steeply graded junction), electric field concentration tends to occur at the end portion of the front portion, due to the curvature of the end portion, as a result of which a so-called local breakdown tends easily to occur.
In order to solve the above problem, a guard ring is formed. By arranging the guard ring, a p-n junction having a slow concentration gradient (to be referred to as a moderately graded junction hereinafter) is formed at the end portion of the front portion of the steeply graded junction.
In the moderately graded junction, a breakdown voltage is generally high and an electric field concentration effect due to the curvature is insufficient. Therefore, the guard ring can suppress the local breakdown. At this time, the depth of the moderately graded junction must be slightly larger than that of the steeply graded junction. If the steeply graded junction is too close to the moderately graded junction, the effect of the moderately graded junction is decreased. In addition, a method of equivalently increasing the curvature of an impurity doped interface can effectively suppress the local breakdown.
A conventional APD wherein a guard ring is formed will be described hereinafter with reference to FIGS. 1 to 4.
FIG. 1 is a sectional view showing an example of a conventional APD disclosed in Lecture Papers of the 46th Lectures of the Japan Society of Applied Physics, 1985, Autumn. In FIG. 1, an n-InP buffer layer 2, an n.sup.- -GaInAs light absorbing layer 3, an n.sup.- -GaInAsP (composition having an absorption edge wavelength of 1.3 .mu.m) intermediate layer 4, an n-InP avalanche multiplication/electric field relaxation layer 5, an n.sup.- -InP window layer 6, a p-type guard ring 8 for forming a moderately graded junction, a p-type light receiving region 9 for forming a steeply graded junction, an insulating layer 11, and a non-reflection coating layer 12 are sequentially formed on an n-InP substrate 1. An electrode 13 is formed on an exposing surface of the light detection layer 9, and an electrode 14 is formed on a lower surface of the substrate 1.
In the APD having the above structure, a depletion layer extends under the p-type regions 8 and 9, and carriers generated in the light absorbing layer 3 are guided to a high electric field region concentrated in the p-n junction region and are subjected to avalanche multiplication. In the optical communication APD, in order to minimize a decrease in response speed due to the avalanche multiplication, the light detecting region 9 is generally formed close to the n-InP layer 5 to decrease the width of the high electric field region. Therefore, the junction defined by the guard ring 8 is formed to extend in the n-InP layer 5. However, since the impurity concentration of the n-InP layer 5 is relatively high, an effect of the moderately graded junction on the interface of the guard ring 8 tends to be insufficient, thus easily causing local breakdown. In the prior art shown in FIG. 1, a shallow guard ring is expanded in a longitudinal direction to prevent the local breakdown. This is equivalent to an increase in curvature of the peripheral portion of the guard ring. In such a prior art, however, the electric field value of the light detection layer 3 tends to increase because the moderately graded junction is deep under the steeply graded junction, and a current generated in a hetero-interface or a dark current due to a tunnel current in the light detection layer 3 tends to be increased. The above problems are posed when the impurity concentration of the n-InP layer 5 is increased and the thickness thereof is decreased to achieve a high-speed operation.
FIG. 2 is a sectional view showing a prior art wherein the shape of the interface portion of the steeply graded junction is changed and the equivalent curvature of the interface is increased. This prior art is disclosed in the Institute of Electronics and Communication Engineers of Japan, the National Meeting, Lecture Papers C - 172, 1988, Autumn. In this prior art, a guard ring providing the moderately graded junction is not formed, but only the shape of the interface portion of the steeply graded junction prevents local breakdown. This modernately grated junction is formed by thermal diffusion of Zn, Cd, or the like. A central light detecting region (projection) is formed by selective forced diffusion of an impurity. As a result, a high electric field region is formed in the central projection, and the electric field concentration in the projection interface is decreased.
In such a structure, a dark current is not unnecessarily increased, as compared with the prior art in FIG. 1, and an element almost free from the dark current can be obtained. In such a structure, however, in order to relax the curvature of the peripheral front portion of the steeply graded junction, relatively deep diffusion (about 6 .mu.m) is performed, and the thickness of the n-InP layer 6 must be increased as compared with that in diffusion (diffusion depth: 1 to 2 .mu.m) using a normal moderately graded junction (guard ring). For this reason, cost is undesirably increased to perform crystal growth. In addition, controllability of the diffusion depth is degraded because of deep diffusion, and flatness of the front portion of the diffusion region is easily damaged. Therefore, the nonuniformity of the light detection sensitivity undesirably tends to occur over the light detection region. In addition, when the front portion of the diffusion region approaches the n-InP layer 5 in order to achieve a high-speed operation of the device, local breakdown in the front peripheral portion tends to occur again. As a result, this structure is not suitable for a high-speed APD.
FIG. 3 shows a prior art wherein a part of the n-InP layer 5 on which the guard ring is to be formed is removed, and another n.sup.- -InP layer 6 is grown again. This prior art is disclosed in Japanese Patent Disclosure No. 61-220481. In this prior art, the guard ring providing the moderately graded junction is formed in the front peripheral portion of the steeply graded junction. However, since the thickness of the n.sup.- -InP layer 6 grown again is relatively large, the moderately graded junction can hardly affect the light detection layer 3. In addition, since the diffusion depth is normal and the shape of the diffusion front can be deformed at a regrowth interface, a substantially ideal guard ring structure can be obtained by performing optimization. Such an excellent structure is suitable for a high-speed APD, and problems on its characteristics can be substantially solved.
The drawbacks of the above structure are, however, caused by the such problems in a manufacturing method that a regrowth interface exists in the high electric field region and double crystal growth must be performed. More specifically, a perfect regrowth interface is required to withstand a high electric field wherein avalanche multiplication can be performed. Therefore, crystal regrowth almost free from an interface level is required. For this reason, the following method has been employed. That is so-called "melt back" (etching in a growth furnace by crystal growth melt) is slightly performed by an LPE (liquid phase epitaxy) to clean the surface. By employing this method, a substantially perfect regrowth interface can be obtained. However, since the LPE is employed as a regrowth method, the following problems are posed. For example, a wafer size is limited, the flatness of the regrowth interface is easily damaged, and the number of factors to reduce a manufacturing yield is increased. In addition, since the thickness of the layer formed by the second crystal growth is large, i.e., 2 to 3 .mu.m, cost is undesirably increased because of the second crystal growth.
FIG. 4 shows a prior art having the above advantages of the prior arts shown in FIGS. 1 and 2. This prior art is disclosed in Japanese Patent Disclosure No. 61-191082. In this prior art, the thickness of the n.sup.- -InP layer 6 is slightly increased, and the central light detection region portion is removed by etching or ion milling to obtain a smooth profile. Thereafter, a moderately graded junction is formed. The guard ring 8 providing the moderately graded junction is formed in the front peripheral portion of the steeply graded junction. In the structure according to this prior art, the n.sup.- -InP layer obtained by regrowth is not required, and the diffusion depth of the steeply graded junction may be relatively small. Therefore, the above problems are not posed. However, in such a prior art, the extremely high-precision controllability of etching or ion milling is required, so that the manufacturing yield becomes low.